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Found 10 entries in the Bibliography.
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| 2020 |
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TWINS Observations of the Dynamics of Ring Currents Ion Spectra on 17th March and 7th October 2015 Direct comparisons between RBSP (Van Allen Probes or Radiation Belt Storm Probes) and TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) for the main phase of two storms, 17th March and 7th October 2015, showed agreement between the in–situ ion measurements and the ion spectra from the deconvolved energetic neutral atom (ENA) measurements, except when O+ ions were significant. Spatial evolution of individual energy peaks in the ion spectra are studied using TWINS data. O+ ions are seen to result in intense peaks at 5–10 keV/amu in the TWINS ion spectra. These ion populations are confined to low L shells (L < 5) and localized in the pre midnight sector. When H+ ions are significant, the low energy peaks ( < 25 keV/amu) are found to be less intense than the high energy peaks ( > 25 keV/amu), located at L > 4 and localized within the premidnight sector. During times of rapidly varying AE indices, two spatially distinct peaks, between 3–5RE and 6–8RE, are observed for the ions with energies > 25 keV/amu. The outer peak appears for a few hours and fades while the inner peak is more stable. These structures are found to be consistent with particle injections observed in the RBSP data. When double peaked structures are swept off, low energy ions accumulate in the pre midnight to midnight sectors whereas high energy ions are located pre to post midnight sectors. Faster drift orbits of > 25 keV/amu ions may cause this kind of distribution.This article is protected by copyright. All rights reserved. Shekhar, S.; Perez, J.; Ferradas, C.; Published by: Journal of Geophysical Research: Space Physics Published on: 12/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA028156 Ring Currents; Magnetosphere; energy dependent drift; ion nose; Substorm Injections; Ion Spectra; Van Allen Probes |
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Simultaneously Formed Wedge-Like Structures of Different Ion Species Deep in the Inner Magnetosphere In this study, ion data from the Helium, Oxygen, Proton, and Electron (HOPE) spectrometers onboard Van Allen Probes reveal the existence of wedge-like structures of O+, He+, and H+ ions deep in the inner magnetosphere. The behaviors of the wedge-like structures in terms of temporal evolution, spatial distribution, upper energy limit, as well as dependence on solar wind and different geomagnetic indices are investigated from both event studies of several consecutive orbits on 3 February 2013 and the subsequent statistical analyses using 4 years of data. Unlike the dominant distribution at –8 in the dayside observed by the polar orbit satellites in previous studies, the wedge-like structures deep in the equatorial plane of the inner magnetosphere are found mostly at the Mcllwain L shells of –5 and have a preferential location in the duskside and nightside. The O+ and He+ structures can extend to smaller L shells with higher upper energy limits than the H+ structures, while the upper energy limits of all these particle species show a similar variation tendency with respect to magnetic local time (MLT) and L. Observations indicate that these wedge-like structures are probably attributed to fresh substorm injections from the outer region. Ren, Jie; Zong, Q.; Yue, C.; Zhou, X.; Fu, S; Spence, H.; Funsten, H.; Published by: Journal of Geophysical Research: Space Physics Published on: 11/2020 YEAR: 2020 DOI: https://doi.org/10.1029/2020JA028192 wedge-like structures; Ring current ions; inner magnetosphere; Substorm Injections; Van Allen Probes |
| 2018 |
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To better understand rapid enhancements of the seed populations (hundreds of keV electrons) in the heart of the Earth\textquoterights outer radiation belt (L* ~ 3.5\textendash5.0) during different geomagnetic activities, we investigate three enhancement events measured by Van Allen Probes in detail. Observations of the fluxes and the pitch angle distributions of energetic electrons are analyzed to determine rapid enhancements of the seed populations. Our study shows that three specified processes associated with substorm electron injections can lead to rapid enhancements of the seed populations, and the electron energy increases up to 342 keV. In the first process, substorm electron injections accompanied by the transient and intense substorm electric fields can directly lead to rapid enhancements of the seed populations in the heart of the outer radiation belt. In the second process, the substorm injected electrons are first trapped in the outer radiation belt and subsequently transported into L* < 4.5 by the convection electric field. In the third process, the lower energy electrons are first injected at L* ~ 5.3 and then undergo drift resonance with ultralow-frequency waves. These accelerated electrons by ultralow-frequency waves are further transported into L* < 4.5 due to the convection electric field. This process is consistent with the radial diffusion. Our results suggest that these specified processes are important for understanding the dynamics of the seed populations in the heart of the outer radiation belt. Tang, C.; Xie, X.; Ni, B.; Su, Z.; Reeves, G.; Zhang, J.-C.; Baker, D.; Spence, H.; Funsten, H.; Blake, J.; Wygant, J.; Dai, G; Published by: Journal of Geophysical Research: Space Physics Published on: 06/2018 YEAR: 2018 DOI: 10.1029/2017JA025142 enhanced convection; Substorm Injections; the outer radiation belt; the seed population; ULF waves; Van Allen Probes |
| 2017 |
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Using the particle data measured by Van Allen Probe A from October 2012 to March 2016, we investigate in detail the radiation belt seed population and its association with the relativistic electron dynamics during 74 geomagnetic storms. The period of the storm recovery phase was limited to 72 h. The statistical study shows that geomagnetic storms and substorms play important roles in the radiation belt seed population (336 keV electrons) dynamics. Based on the flux changes of 1 MeV electrons before and after the storm peak, these storm events are divided into two groups of \textquotedblleftlarge flux enhancement\textquotedblright and \textquotedblleftsmall flux enhancement.\textquotedblright For large flux enhancement storm events, the correlation coefficients between the peak flux location of the seed population and those of relativistic electrons (592 keV, 1 MeV, 1.8 MeV, and 2.1 MeV) during the storm recovery phase decrease with electron kinetic energy, being 0.92, 0.68, 0.49, and 0.39, respectively. The correlation coefficients between the peak flux of the seed population and those of relativistic electrons are 0.92, 0.81, 0.75, and 0.73. For small flux enhancement storm events, the correlation coefficients between the peak flux location of the seed population and those of relativistic electrons are relatively smaller, while the peak flux of the seed population is well correlated with those of relativistic electrons (correlation coefficients >0.84). It is suggested that during geomagnetic storms there is a good correlation between the seed population and <=1 MeV electrons and the seed population is important to the relativistic electron dynamics. Tang, C.; Wang, Y.; Ni, B.; Zhang, J.-C.; Reeves, G.; Su, Z.; Baker, D.; Spence, H.; Funsten, H.; Blake, J.; Published by: Journal of Geophysical Research: Space Physics Published on: 05/2017 YEAR: 2017 DOI: 10.1002/2017JA023905 relativistic electrons; Substorm Injections; the outer radiation belt; the seed population; Van Allen Probes |
| 2014 |
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We simulate substorm injections observed by the Van Allen Probes during the 17 March 2013 storm using a self-consistent coupling between the ring current model RAM-SCB and the global MHD model BATS-R-US. This is a significant advancement compared to previous studies that used artificially imposed electromagnetic field pulses to mimic substorm dipolarization and associated inductive electric field. Several substorm dipolarizations and injections are reproduced in the MHD model, in agreement with the timing of shape changes in the AE/AL index. The associated inductive electric field transports plasma sheet plasma to geostationary altitudes, providing the boundary plasma source to the ring current model. It is found that impulsive plasma sheet injections, together with a large-scale convection electric field, are necessary to develop a strong ring current. Comparisons with Van Allen Probes observations show that our model reasonably well captures dispersed electron injections and the global Dst index. Yu, Yiqun; Jordanova, Vania; Welling, Dan; Larsen, Brian; Claudepierre, Seth; Kletzing, Craig; Published by: Geophysical Research Letters Published on: 02/2014 YEAR: 2014 DOI: 10.1002/2014GL059322 ring current dynamics; self-consistent treatment of fields and plasma; Substorm Injections; Van Allen Probes |
| 2010 |
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We study the relationship between the electron injection and the chorus waves during a substorm event on 23 March 2007. The chorus waves were detected at high geomagnetic latitude (\~70\textdegreeS) Antarctic observatories in the range of 0600\textendash0900 h in magnetic local time (MLT). Electrons drifting from the injection event were measured by two LANL spacecraft at 0300 and 0900 MLT. The mapping of auroral brightening areas to the magnetic equator shows that the injection occurred in an MLT range of 2200\textendash2400. This estimate is consistent with observations by the THEMIS A, B, and D spacecraft (which were located at 2100 MLT and did not observe electron injections). Our backward model tracing from the magnetic equator near the dawnside magnetopause (which magnetically connects to the Antarctic observatories) also supports the deduced injection region. Since chorus waves are believed to be generated through the electron cyclotron instability by an anisotropic temperature distribution, we examine, by performing forward model tracing, whether the electrons injected during this substorm form a pancake-like pitch angle distribution when they arrive near the dawn-side magnetopause. We find that the onset time of the modeled pitch angle anisotropy is consistent with that of the observed chorus waves. We conclude that the development of the anisotropy is due to particle drift shell splitting. Min, Kyungguk; Lee, Jeongwoo; Keika, Kunihiro; Published by: American Geophysical Union Published on: 10/2010 YEAR: 2010 DOI: 10.1029/2010JA015474 chorus and substorm; electron drift; RBSP; Substorm Injections; Van Allen Probes |
| 2005 |
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One of the main questions concerning radiation belt research is the origin of very high energy (>1 MeV) electrons following many space storms. Under the hypothesis that the plasma sheet electron population is the source of these electrons, which are convected to the outer radiation belt region during substorms, we estimate the flux of particles generated at geosynchronous orbit. We use the test particle method of following guiding center electrons as they drift in the electromagnetic fields during substorm dipolarization. The dipolarization pulse model electromagnetic fields are taken from the Li et al. (1998) substorm particle injection model. We find that a substorm dipolarization can produce enough electrons within geosynchronous orbit to account for the electrons seen following storms. To do this, we compute transport ratios of plasma sheet electrons, that is, the relative ratio of plasma sheet electrons that are transported and trapped in the inner magnetosphere during substorms, as well as the change in energy of the electrons. Since high fluxes of MeV electrons are only seen following storms and not isolated substorms, it is likely that these electrons may serve as a source population for other energization mechanisms which accelerate the electrons to MeV energies. Furthermore, we do parametric studies of the dipolarization model to understand physically what conditions enable the generation of this source population. Published by: Journal of Geophysical Research Published on: 07/2005 YEAR: 2005 DOI: 10.1029/2004JA010511 |
| 2001 |
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The great March 1991 magnetic storm and the immediately preceding solar energetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate here, using data from eight satellites, the very large delayed buildup of relativistic electron flux in the outer zone during a 1.5-day period beginning 2 days after onset of the main phase of this storm. A notable feature of the March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower-energy substorm-injected electron flux and the relativistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and local noon shows evidence that both classes of electrons arrive at geosynchronous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric field transports not only the usual (50\textendash300 keV) substorm electrons but also the relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A simplified calculation of the electron ε \texttimes B and gradient/curvature drifts indicates that sufficiently strong substorm dipolarization inductive electric fields (≳ 10 mV/m) could achieve this, provided sufficient relativistic electrons are present in the source region. Consistent with this interpretation, we find that the injected relativistic electrons have a pitch angle distribution that is markedly peaked perpendicular to the magnetic field. Furthermore, the equatorial phase space density at geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during this time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a strong addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for this magnetic storm. Ingraham, J.; Cayton, T.; Belian, R.; Christensen, R.; Friedel, R.; Meier, M.; Reeves, G.; Takahashi, K; Published by: Journal of Geophysical Research Published on: 11/2001 YEAR: 2001 DOI: 10.1029/2000JA000458 |
| 2000 |
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The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere are investigated using data from the CRRES satellite. Equatorial electron distributions and concomitant wave spectra outside the plasmapause on the nightside of the Earth are studied as a function of time since injection determined from the auroral-electrojet index (AE). The electron cyclotron harmonic (ECH) wave amplitudes are shown to be very sensitive to small modeling errors in the location of the magnetic equator. They are best understood at the ECH equator, defined by the local maximum in the ECH wave activity in the vicinity of the nominal magnetic equator, suggesting that the ECH equator is a better measure of the location of the true equator. Strong ECH and whistler mode wave amplitudes are associated with the injected distributions and at the ECH equator, in the region 6.0 <= L < 7.0, exponential fits reveal wave amplitude decay time constants of 6.3\textpm1.2 and 4.6\textpm0.7 hours, respectively. Pancake electron distributions are seen to develop from injected distributions that are nearly isotropic in velocity space and, in this region, are seen to form on a similar timescale of approximately 4 hours suggesting that both wave types are involved in their production. The timescale for pancake production and wave decay is comparable with the average time interval between substorm events so that the wave-particle interactions are almost continually present in this region leading to a continual supply of electrons to power the diffuse aurora. In the region 3.8 <= L < 6.0 the timescale for wave decay at the ECH equator is 2.3 \textpm 0.6 and 1.1 \textpm 0.2 hours for ECH waves and whistler mode waves respectively, although the pancakes in this region show no clear evolution as a function of time. Meredith, Nigel; Horne, Richard; Johnstone, Alan; Anderson, Roger; Published by: Journal of Geophysical Research Published on: 06/2000 YEAR: 2000 DOI: 10.1029/2000JA900010 |
| 1998 |
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Substorm electron injections: Geosynchronous observations and test particle simulations We investigate electron acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous observations and test-electron orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. This complements an earlier investigation of test protons [Birn et al., 1997b]. In the present paper we consider equatorial orbits only, using the gyrocenter drift approximation. It turns out that this approximation is valid for electrons prior to and during the flux rises observed in the near tail region of the model at all energies considered (\~ 100 eV to 1 MeV). The test particle model reproduces major observed characteristics: a fast flux rise, comparable to that of the ions, and the existence of five categories of dispersionless events, typical for observations at different local times. They consist of dispersionless injections of ions or electrons without accompanying injections of the other species, delayed electron injections and delayed ion injections, and simultaneous two-species injections. As postulated from observations [Birn et al., 1997a], these categories can be attributed to a dawn-dusk displacement of the ion and electron injection boundaries in combination with an earthward motion or expansion. The simulated electron injection region extends farther toward dusk at lower energies (say, below 40 keV) than at higher energies. This explains the existence of observed energetic ion injections that are accompanied by electron flux increases at the lower energies but not by an energetic electron injection at energies above 50 keV. The simulated distributions show that flux increases are limited in energy, as observed. The reason for this limitation and for the differences between the injection regions at different energies is the localization in the dawn-dusk direction of the tail collapse and the associated cross-tail electric field, in combination with a difference in the relative importance of E \texttimes B drift and gradient drifts at different energies. The results demonstrate that the collapsing field region earthward of the neutral line appears to be more significant than the neutral line itself for the acceleration of electrons, particularly for the initial rise of the fluxes and the injection boundary. This is similar to the result obtained for test ions [Birn et al., 1997b]. Birn, J.; Thomsen, M.; Borovsky, J.; Reeves, G.; McComas, D.; Belian, R.; Hesse, M.; Published by: Journal of Geophysical Research Published on: 05/1998 YEAR: 1998 DOI: 10.1029/97JA02635 |
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